How to export MTree to UE4?
The default leaves generated in the Mtree node system are a Python scripted
particle system. Unreal reads each particle as a separate object. Therefore, we need to combine the particles and the tree trunk/roots/branches etc. into a single mesh for Unreal.
There a few steps and the (processing) time required varies depending on the complexity of your generated tree(s).
I also strongly suggest texturing your tree and leaves before beginning this process

Select just the leaves and navigate to the modifier properties:



Select convert, this will make each leaf particle a separate object, and hit apply on the modifier settings:



Shift +  LMB the trunk of the tree and parent the leaves to it with Ctrl + P and select Object (Keep Transforms). Again, however long the parenting process takes is dependent on the complexity of your tree trunk (roots, branches, twigs etc.):



When that is finally done,  RMB on the parented tree+leaves and select Convert to Mesh, this will be much quicker than parenting:



Now (with everything still selected) we right click again and select Join.
If your tree object still have a separate leaf object, then select both your tree object and leaf object, right click on the leaves specifically and select Join again.
Voila! All the leaves and the tree are now a single mesh, ready to be imported to Unreal:


In my experience, the materials don't always import correctly therefore playing around with the import settings and material editor is still required, but our base tree is now in Unreal. This method could also work with Epic's Blender-Unreal import plugin that you can search for on their github.